New in Symfony 2.4: Easier debugging of not-found resources

When a 404 occurs in a web application, knowing the root cause helps fixing
the problem: is it someone trying to hack your website by trying different
well-known URLs? Is it a dead-link on your website? or on another one? If the
404 comes from your website, fixing it should be relatively easy.

The only source of information about the cause is the HTTP Referer header.
And even if it cannot be trusted, it can help nonetheless. That's why as of
Symfony 2.4, the HTTP Referer is added in the NotFoundHttpException
exception message thrown by the router listener.